OVERALL SUMMARY/ABSTRACT Allogeneic BMT is an increasingly important treatment for hematologic cancers such as leukemia and lymphoma. GVHD remains a major cause of morbidity and mortality after allogeneic BMT and prevents this curative therapy from wider application. Gastrointestinal (GI) GVHD is the most difficult of the three principal GVHD target organs to treat successfully. The three projects of this PPG focus on GI GVHD during this next cycle, and it provides powerful unifying and integrating themes for the entire PPG. Project 1 addresses significant lacunae in our understanding of the role of intestinal epithelial cells (IECs) and their ability to resist donor T-cell-mediated damage in the context of GI microbiota. The microbial metabolite butyrate enhances histone acetylation and modulates IEC sensitivity to GVHD damage. This project will pursue three specific aims (SA): 1) To determine the pathways of sensing of microbial metabolite butyrate by epithelial cells and its impact on GVHD; 2) To analyze the molecular mechanisms for butyrate mediated resistance of IECs after allogeneic BMT. 3) To determine the impact of butyrate administration on GVL. SA 2 has been revised to include a subaim analyzing the effect of antibiotics on HDAC inhibition of GVHD. SA 2 makes extensive use of new services provided by Cores 2 and 3. Both SA 2 and SA 3 are closely coordinated with similar new SA's in Project 2, which uses the same antibiotic regimens and tumor models. Project 2 will investigate Regenerative 3 alpha (REG3A), a biomarker that this PPG has validated as specific for GI GVHD. This project has generated extensive preliminary data to support its central hypothesis that REG3A or (its mouse homologue Reg3?) functions as a master regulator of GVHD pathophysiology. Project 2 will pursue three specific aims: 1) To define the mechanisms by which Reg3? reduces GVHD, 2) To determine if enterococcus drives GVHD, and 3) To determine the effects of both antibiotic treatment and therapeutic probiotics on the GVHD severity. SA 1 has been revised to include a subaim investigating the effect of Reg3? on GVL effects. Project 1 uses the same two tumor bioluminescent models, and donor/recipient strain combinations as Project 1. SA 2 is entirely new and examines the mechanism by which shifts in the microbiome drive GVHD, using sophisticated gnotobiotic model systems and metagenomic sequencing. SA 3 includes a new subaim investigating the mechanism by which systemic antibiotics increase the severity of GVHD. Project 3 has developed and validated a novel scoring system of GVHD at its onset using biomarkers (including REG3A) that categorizes patients according to predicted six month NRM. This novel grading system identifies high risk GVHD at onset independently of clinical presentation. It will test the central hypothesis that risk-adapted GVHD treatment will reduce the development of steroid-refractory GI GVHD in two specific aims: 1) To conduct a clinical trial using natalizumab and to improve day 28 complete response rates in patients with high risk GVHD, and 2) To develop a biomarker algorithm at day 7 post-BMT that predicts severe GVHD. The three projects are supported by three cores. (1) Administration and Biostatistics; (2) Microbiome and Therapeutic Probiotics and (3) Pathology, Clinical Database, Biorepository and Biomarkers. We have enhanced several several elements of Cores 1 and 3 and completely restructured Core 2, initiating a major collaboration with the new core director in germ free and gnotobiotic mouse models, together with advanced metagenomics sequencing of microbiome. This PPG will have a positive impact because of its translational dynamic, with improved understanding of the biology of GI GVHD able to facilitate novel targeted therapies. In this revision, incresead synergies derive from the closely coordinated studies on GVL and the effects of systemic antibiotics in Project 1 and 2. Both projects use the innovative services in microbiome analysis offered by Core 2 and all three project share key points of comparison in GI histology using the new quantitative pathologic services offered by Core 3.